Drill bit

ABSTRACT

Flat drill bit for drilling holes in wood, comprising a plate (12) with two longitudinal sides (16,17), two main radial edges (14,15) and a pilot point with two converging pilot edges (18,19), where main radial edges, pilot edges and short lateral edges (24,26) being short portions of the longitudinal sides all have positive rake angle, produced by grooves parallel to the edges facing forwards during rotation. For larger diameters the pilot point has two converging pilot edges and two parallel sides (27,28).

BACKGROUND

Flat bits are commonly used for drilling of holes with diameter 10 to 40mm (3/8 to 1 1/2 inch) in wood with electric hand drills. Since they arenot self-feeding it is easy to control the penetration into theworkpiece, and they are thus used above all for shallow holes and forthrough holes in relatively thin workpieces.

Simple flat bits, described in the U.S. Pat. Nos. 2,794,468 and4,066,379 consist of a shaft and a flat part made as a plate withsubstantially even thickness, a triangular pilot point, twosubstantially radial main cutting edges and two substantially parallelnon-cutting longitudinal edges. The shaft is attached to the flat partby welding, by brazing or by a threaded connection. It is in generalrequired that flat bits should be possible to resharpen by filing.

The purpose of the pilot point is to commence the drilling of the holeat a well defined center, to stabilize the drill bit when the radialedges start to cut in order to get a clean non-splintered perimeter atthe entry side, and when a through hole is to be drilled to make a pilothole with small diameter in order to locate and guide the drill when thelast part of the hole is drilled from the reverse side of the workpiece.

The purpose of the main radial edges is to cut and transport chips witha low and even force, and to form the major part of the bottom surfaceof a dead end hole.

These different purposes set partly conflicting requirements for thedesign of the flat bit, and it has been difficult to satisfy all to anacceptable extent, in spite of many improvements on the original design.A pilot point with even thickness offers acceptable stability but has alarge negative rake angle, leading to high cutting force, unsatisfactorychip transport and overheating when drilling in resinous or hard wood.It will also deviate to the side if the hardness of the workpiece is nothomogeneous, as in drilling parallel to the fibers or near a knot. Thesame problems but less serious are found with pyramid-shaped points. Apilot point which has grooves along the edge according to U.S. Pat. No.2,782,824 has positive rake angle, lower cutting force and betterdefined position, but will not stabilize the drill well enough when theforce on the the main cutting edges varies. All triangular pilot pointswill also cause a parallel error and lack of accuracy when a throughhole is drilled at some other angle than normal to the surface of theworkpiece.

A drill bit with a pilot point with partly parallel sides is describedin U.S. Pat. No. 2,543,206 and provides good stability and good guidingin through holes, but other features of this drill bit restrict it todrilling with high rpm, slow penetration and limited diameter. Pilotpoints where the width decreases near the main radial edges are knownfrom U.S. Pat. No. 3,920,350 and lead to somewhat less cutting force butless stability.

The main radial cutting edges are normally located at an angle of 70 to90 degrees to the drill axis, 90 degrees giving the lowest cutting forcewhen drilling at right angle to the fibres but a greater risk of unevenor splintered perimeter except when drilling with very low feed rate. Asmaller angle produces a neater perimeter, less vibrations when enteringthe workpiece and has less requirements for stabilizing, but needs morecutting force and wears more rapidly. If grooves are made parallel tothe main radial edges the rake angle will be positive and the cuttingforce lower, but the perimeter less neat. It is known from the patentU.S. Pat. No. 4,682,917 to make spurs protruding from the outer ends ofthe main radial edges in order to sever the fibers before they aretouched by the main radial edges. This allows main radial edges at 90degrees to the drill axis and positive rake angle, producing neat holeswith low cutting force, but this type of drill cannot easily beresharpened.

PURPOSE OF THE INVENTION

The invention is a flat drill bit which better than previous types canfill the different requirements. A new design of the outer ends of themain radial edges makes it possible to achieve at the same time asatisfactory hole perimeter, simple resharpening, and low and smoothcutting forces which are simple to stabilize with the pilot point whenthe main radial edges start cutting. The radial component of the cuttingforce at each main radial edge can be made so small that there is nodifficulty in producing holes overlapping each other or the workpieceedge.

This allows a pilot point with positive rake angle in order to give lowcutting force, or for large diameters a combination of positive rakeangle and partially parallel sides, to give excellent stability andprecision for through holes.

The new design of the outer ends of the main radial edges means that aminor portion of the substantially parallel longitudinal sides of theplate are made as cutting edges with positive rake angle, but withoutany extending spurs. This will make it possible to balance not only theradial forces from the corner, but also the radial force componentappearing along the main radial edge which due to the finite thicknessof the plate is not perpendicular to its local direction of motion.

DESCRIPTION

The invention is described with reference to FIGS. 1 and 2, showingalternative embodiments of flat drill bits according to the invention,FIG. 3 showing a cross-section through the pilot point and FIG. 4showing a cross-section close to the main radial edges.

A flat drill bit according to the invention comprises a plate (12) and ashank (10) the free end (11) of which can be designed for easy clampingby a drill chuck, with three or six flat surfaces. The plate has twolongitudinal sides (16,17) which are substantially parallel orpreferrably slightly converging towards the shank end, forming an anglenot over 2 degrees with the axis of the shank (10), two main radialedges (14,15) forming an angle of from 65 to 90 degrees, preferrablyfrom 70 to 85 degrees, with the axis of the shank (10), and a pilotpoint with two converging pilot edges (18,19).

On each face of the plate (12) the main radial edge (14) and the pilotedge (18) moving forward are provided with grooves (20,21) milled,ground or swaged along the edges to make the rake angle positive, atleast 5 degrees at the main radial edge (14). The longitudinal sides ofthe plate are blanked or ground at an angle to the faces to provide arelief angle.

The width and depth of the grooves (20,21) are substantially constantalong the whole edges (14,18) except in a small region near the end ofthe main radial edge (14) at the longitudinal side (16), where the depthof the groove is rapidly reduced to create a short lateral edge (26)with positive rake angle. The lateral edge (26) meets the main radialedge (14) in a corner with a slightly acute angle, but without forming aprotruding spur. The length of the lateral edge (26) can be equal to thewidth of the groove (21) as shown in FIG. 1, or the lateral edge (24)can be slightly longer by provision of a groove (25) along a portion ofthe longitudinal side (16).

The groove (20,21,25) section is preferrably unsymmetric with the deeperparts farther from the edge than half the width of the groove. The drillbit can then be resharpened more times before the rake angle becomesdisadvantageous.

In the alternative version shown in FIG. 2, which is preferrably usedfor diameters above 20 mm (3/4 inch) the pilot point is made with bothconverging pilot edges (18,19) and parallel sides (27,28), with theparallel sides preferrably made with zero relief angle or slightlyrounded to give maximal stability against unbalanced radial forces atthe main radial edges (14,15) and the lateral edges (24,26). Theparallel sides (27,28) should be so long that they at least to someextent have entered the workpiece before the main radial edges (14,15).

When drilling a hole with a drill bit according to FIG. 2 the first partof the hole is drilled from the front of the workpiece far enough thatthe parallel sides (27,28) of the pilot point have penetrated thereverse side of the workpiece, and the last part of the hole from thereverse side. Since the parallel sides will then follow a a cylindricalpilot hole the first and last parts of the hole will match perfectly andthe hole perimeter on the reverse side will be free of splinters andchipping.

When drilling a hole at an oblique angle to the workpiece surface thecritical moment is when the outer end of the main radial edge (14)starts to cut into the surface. If the cutting force at this end islarger than the forces at the pilot point, there is a risk that thedrill bit will rotate around some other axis than the pilot point, anddamage the workpiece surface. The risk is especially great if the mainradial edge (14) joins a non-grooved longitudinal side (16). A drill bitaccording to the invention has positive rake angle at the main radialedge (14) as well as at the first cutting edge part (24,26) of thelongitudinal side (16). The cutting force is then much lower, and easierto balance with forces on the pilot point. For large diameter drills thehigh tangential velocity will increase the cutting force at the outerend of the main radial edge (14), and to balance this with higher forcesat the pilot point it is advantageous to make the pilot point withpartly parallel sides (27,28) according to FIG. 2. A very great forcewould then be needed to tear the pilot point from its hole in the workpiece.

We claim:
 1. Flat drill bit for drilling holes in wood, comprising:aplate with two substantially parallel longitudinal sides, two mainradial edges which have outer ends forming acute angles with thelongitudinal sides without protruding spurs, short lateral edges whichare portions of the longitudinal sides, a pilot point which has twoconverging pilot edges along at least part of its length and having apositive rake angle at the pilot edges, at the main radial edges and atthe short lateral edges, said positive rake angle being produced bygrooves along the edges facing forwards during rotation.
 2. Flat drillbit according to claim 1, wherein the length of the lateral edges is notless than the width of the groove along the main radial edge and is notmore than half the length of the longitudinal sides.
 3. Flat drill bitaccording to claim 1, wherein the pilot point has two converging pilotedges and two parallel sides.
 4. Flat drill bit according to claim 3,wherein the parallel sides are made without clearance angle.
 5. Flatdrill bit according to claim 3, wherein the parallel sides are rounded.